Electrode materials for electrochemical cells and method of making same

ABSTRACT

A method for preparing a carbon material for use as an electrode, such as the anode (30) of an electrochemical cell (10). The carbon is fabricated in a heating process from a plurality multifunctional organic monomers selected from first and second groups of monomers. Electrodes so fabricated may be incorporated into electrochemical cells (10) as the anode (20) thereof.

This is a Divisional application under §1.60 of pending U.S. patentapplication Ser. No. 08/575,653 filed Dec. 20, 1995 and assigned toMotorola, Inc.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Ser. No. 08/534,427, filed Sep. 27,1995, in the names of Zhang, et al, and assigned to Motorola, Inc., thedisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates in general to the field of electrodes andelectrode materials for electrochemical cells, and in addition, tomethods of synthesizing said electrodes and electrode materials.

BACKGROUND OF THE INVENTION

As electronic devices and other electrical apparatuses increasinglybecome portable, advances must be made in energy storage systems toenable such portability. Indeed, it is often the case with currentelectronics technology that the limiting factor to portability of agiven device is the size and weight of the associated energy storagedevice. Obviously, a small energy storage device may be fabricated for agiven electrical device, but at the cost of energy capacity. Conversely,a long-lasting energy source can be built, but it is then too large tobe comfortably portable. The result is that the energy source is eithertoo bulky, too heavy, or it doesn't last long enough. The main energystorage device used for portable electronics is the electrochemicalbattery cell, and less frequently, the electrochemical capacitor.

Numerous different battery systems have been proposed for use over theyears. Early rechargeable battery systems included lead-acid, andnickel-cadmium (Nicad), each of which have enjoyed considerable successin the marketplace. Lead-acid batteries, because of their ruggedness anddurability, have been the battery of choice in automotive and heavyindustrial applications. Conversely, Nicads have been preferred forsmaller or portable applications. More recently, nickel metal hydridesystems (NiMH) have found increasing acceptance for both large and smallapplications.

Notwithstanding the success of the aforementioned battery systems, othernew batteries are appearing on the horizon which offer the promise ofbetter capacity, better power density, and longer cycle life as comparedwith the current state of the art. The first such system to reach themarket is the lithium ion battery, which is already finding its way intoconsumer products. Lithium polymer batteries are also receivingconsiderable attention, though have not yet reached the market.

Lithium ion batteries in general include a positive electrode or cathodefabricated of a transition metal oxide material, and a negativeelectrode or anode fabricated of an activated carbon material such asgraphite or petroleum coke. New materials for both electrodes have beeninvestigated intensely because of their high potential gravimetricenergy density. To date, however, most of the attention has been focusedon the transition metal oxide electrode.

Activated carbon materials are routinely prepared by using difunctionalmonomers as polymer precursors. Examples of such precursors includeresins of furfuryl alcohol, phenol, formaldehyde, acetone-furfural, orfurfural alcohol-phenol copolymer. These precursors are disclosed in,for example, U.S. Pat. No. 5,378,561, to Furukawa, et al. Otherprecursors include polyacrylonitrile and rayon polymers, as disclosed inJenkins, et al, Polymeric Carbons-Carbon Fibre, Glass and Char,Cambridge University Press, Cambridge, England (1976). These precursorsare then subjected to a process of curing and carbonizing, usually veryslowly, and at temperatures of up to 2,000° C. Two major steps areinvolved in these processes: (1) synthesis of polymer precursors fromdifunctional monomers via wet chemistry; and (2) pyrolysis of theprecursors. The method typically results in a relatively low overallyield due to the two step process. For example, conventional processingof polyacrylonitrile typically yields only about 10% of a usablecarbonaceous material. Further, many impurities may be incorporated intothe carbonaceous material, deleteriously effecting the electrochemicalproperties.

Accordingly, there exists a need for an improved, carbon material foruse in electrochemical cell applications. The material should be easilymanufactured in a simple, high yield method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an electrochemical cellincluding an electrode fabricated of carbon electrode material, inaccordance with the instant invention;

FIG. 2 is a flowchart illustrating the steps for preparing a carbonelectrode material, in accordance with the instant invention;

FIG. 3 is a X-ray diffraction pattern of an amorphous carbon electrodematerial, in accordance with the instant invention;

FIG. 4 is a X-ray diffraction pattern of a second amorphous carbonelectrode material, in accordance with the instant invention;

FIG. 5 is a X-ray diffraction pattern of a third amorphous carbonelectrode material, in accordance with the instant invention;

FIG. 6 is a X-ray diffraction pattern of a fourth amorphous carbonelectrode material, in accordance with the instant invention;

FIG. 7 is a X-ray diffraction pattern of a fifth amorphous carbonelectrode material, in accordance with the instant invention;

FIG. 8 is a X-ray diffraction pattern of a sixth amorphous carbonelectrode material, in accordance with the instant invention;

FIG. 9 is a X-ray diffraction pattern of a seventh amorphous carbonelectrode material, in accordance with the instant invention; and

FIG. 10 is a X-ray diffraction pattern of an eighth amorphous carbonelectrode material, in accordance with the instant invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the specification concludes with claims defining the features ofthe invention that are regarded as novel, it is believed that theinvention will be better understood from a consideration of thefollowing description in conjunction with the drawing figures, in whichlike reference numerals are carried forward.

Referring now to FIG. 1, there is illustrated therein a schematicrepresentation of an electrochemical cell 10 such as a battery or anelectrochemical capacitor, and including an amorphous carbon orcarbon-based electrode fabricated in accordance with the instantinvention. The electrochemical cell includes a positive electrode orcathode 20, a negative electrode or anode 30 and an electrolyte 40disposed therebetween. The cell negative electrode 30 is fabricated of asubstantially amorphous carbon or carbon-based material such as thatdescribed in greater detail hereinbelow. The positive electrode 20 ofthe cell 10 may be fabricated from a lithiated transition metal oxidesuch as are well known in the art. Alternatively, the positive electrodematerial may be fabricated of a material such as that described incommonly assigned, co-pending patent application Ser. No. 08/464,440filed Jun. 5, 1995, in the name of Mao, et al, and entitled "PositiveElectrode Materials for Rechargeable Electrochemical Cells and Method ofMaking Same", the disclosure of which is incorporated herein byreference.

The electrolyte 40 disposed between the electrodes may be a polymerelectrolyte, comprising a polymeric support, having dispersed therein anelectrolyte active species such as, for example, LiClO₄ in propylenecarbonate, or polyethylene oxide impregnated with a lithiated salt.Alternatively, the electrolyte may be similar to that described incommonly assigned, co-pending application Ser. No. 08/518,732 filed Aug.24, 1995 to Oliver, the disclosure of which is incorporated herein byreference. The electrolyte 40 may also act as a separator between thepositive and negative electrodes. The electrolyte may be aqueous,non-aqueous, solid state, gel, or some combination thereof.

In accordance with the instant invention, there is provided asubstantially amorphous carbon or carbon-based material for use as anelectrode in an electrochemical device such as a battery, and a methodfor making said material. The carbon-based materials are substantiallyamorphous, though may be partially or completely crystalline or includecrystalline inclusions if desired, and may include an amount of one ormore modifiers. The exact nature of the modifiers is dependent upon thespecific application contemplated, as will be described below.

Instead of the difunctional monomer precursors used in the prior art,the instant invention uses one or more multifunctional organic monomers.More specifically, the multifunctional organic monomers are divided intotwo groups (Monomer A & Monomer B), at least one monomer being selectedfrom each group. The monomers have the general formulas of: ##STR1##

Where R is selected from the groups of Cl, OH, H, OC_(n) H_(2n+1) (n=1to 10, and preferably 4 or less,), and combinations thereof.

Indeed, where in the prior art a single organic monomeric precursor isused, the instant invention contemplates fabricating the carbonmaterials from two or more than two multifunctional organic monomers, atleast one of which is selected from each of the two groups describedabove. In one preferred embodiment, one of the organic precursormonomers has at least three functional groups, which functional groupsallow for crosslinking in the curing process. More particularly, firstand second multifunctional organic monomers are cured or crosslinked inthe presence of heat and/or a catalyst, as is described in greaterdetail hereinbelow. Following the curing process, the crosslinkedmultifunctional organic monomers are subjected to a solid statecarbonization process described in greater detail hereinbelow. Theresult of the solid state carbonization process is the amorphous carbonelectrode material.

Preferred examples of organic monomers which can be used in connectionwith the instant invention include, from Monomer Group A pyrogallol,phloroglucinol, 1,2,4-benzenetriol, dipentaerythritol, pentaerythritol,trimethyl oylpropane, and combinations thereof. Preferred compounds frommonomer Group B include 1,3,5-benezenetricarbonyl trichloride,terephthaloyl chloride, dimethyl isophthalate, dimethyl terephthlate,isophthaloyl chloride, terephthalic acid, isophthalic acid,1,3,5-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic anhydride,1,2,4,5-benzenetetracarboxylic acid, 1,2,4,5-benzenetetracarboxylicdianhydride, and combinations thereof. Other multifunctional organicmonomers which conform to the formulas described hereinabove may beemployed equally advantageously, without departing from the spirit orscope hereof.

The carbon electrode materials resulting from the processing of theorganic monomer precursors described hereinabove are characterized byX-ray diffraction patterns which individually show a (002) peak, thed-spacings of which (002) peaks (^(d) 002) are between 3.72 Å to 4.20 Å.Further, the optimum peak ratio of the (002) peak to the (100) peak isbetween 1 and 5, and preferably between 2.25 and 2.75. Since the carbonmaterials are essentially amorphous in nature, the X-ray diffractionpeaks are broader than in crystalline materials. This will beillustrated in the examples below.

With respect to the fabrication of carbon electrode materials, it hasbeen found that when the multifunctional organic monomers are heated inthe presence of an acid, the reaction of the monomer may be morecomplete, and may result in an improved yield of the final product.Hence, the amorphous carbon electrode material may be formed with anacid present. Examples of preferred acids include acids selected fromthe group consisting of acetic acid, boric acid, phosphoric acid,p-toluenesulfonic acid, 4-amino benzoic acid, trifluoroacetic acid,benzenesulfonic acid, and combinations thereof. The acid may be presentin amounts between 1 and 25% weight percent.

The carbon material may also include one or more modifiers incorporatedinto the carbon matrix. The modifiers may be selected from the groupconsisting of lithium alloying elements such as Sn, Si, Al, and othersknown in the art, and combinations thereof, and electrode performanceenhancing elements such as B, N, Ti, V, and combinations thereof.

In the preparation of the amorphous carbon material, it is contemplatedthat the multifunctional organic monomers are heated, along with the addcatalyst, in an inert environment. Preferred inert gas environmentsinclude, for example, nitrogen, argon, and helium. The materials areheated at temperatures sufficient to induce a solid state carbonizationof the multifunctional monomers. This process is similar in nature to asublimation process, and occurs at temperatures of less than about 1200°C., and preferably about 1000° C.

The amorphous carbon electrode material is the pyrolytic by-product ofthe multifunctional organic monomers. The multifunctional monomers arecured or polymerized at lower temperatures. Once polymerized, themultifunctional monomers form a cured, crosslinked polymer whichsubsequently carbonizes at higher temperatures to form the carbonelectrode material. As the multifunctional organic monomers generallycontain the elements of carbon, hydrogen, oxygen, and nitrogen invarying combinations, the carbonization process refers to the fact thatthe cross-linked organic precursors decompose, evolving compoundsincluding carbon-oxygen, carbon-hydrogen, hydrogen-oxygen,nitrogen-hydrogen, and other similar compounds. The remaining carbonatoms condense into planar structures terminating predominantly withedge hydrogen atoms.

The fabrication process may be understood from the following: ##STR2##wherein Monomer A and Monomer B are selected from the groups describedabove.

Upon further heating, the cross-linked polymer resulting from the curingprocess decomposes and forms carbon-carbon bonds between the phenylrings of the starting monomers. As the temperature increases up to, forexample, 500°-700° C., the six carbon phenyl rings start to break andform a layered carbon network. The formation of hyperbranched carbonpolymers in the first stage of the process results in moving the monomermolecules physically closer to one another, thus facilitatingcarbonization in the second step of the process. This also accounts, atleast partially, for improved yields as compared to the prior art.

Referring now to FIG. 2, there is illustrated therein a flow chart 100describing the steps for preparing the amorphous carbon electrodematerial described above. The first step illustrated in FIG. 2 is shownin box 102, and comprises the step of selecting appropriatemultifunctional organic monomers from each group. Thereafter, asillustrated in box 103, the two or more organic monomers are cured orcross-linked by heating. Then, as illustrated in box 104, is the step ofselecting the treatment temperature ranges for the solid statecarbonization process for the selected monomers. More particularly, theyield of the amorphous carbon material from a particular multifunctionalmonomer will depend in part on the thermal regime to which the monomeris subjected. Thermogravimetric analysis (TGA) and differential scanningcalorimetry (DSC) each provide means by which to determine theprocessing temperature regime. The results have generally indicated thatthe solid state carbonization process should be a two temperature,one-step heating process.

Specifically, TGA & DSC indicate the temperature at which condensationand reduction of the multifunctional monomers occur. Hence, using suchanalysis as described in commonly assigned, copending application Ser.No. 08/534,427, referred to above, a heating regime is selected.

Returning now to FIG. 2, the next step in the fabrication process offlow chart 100 is illustrated in box 106, and comprises the step ofmixing the multifunctional organic monomers with an acid selected fromthe group of acids described above. The materials should be mixedthoroughly, and further may be dried, as in a drying oven, prior tosubjecting the mixture to the solid state carbonization process. It isto be noted that the organic monomers may be mixed with or without theacid, in the presence of an organic solvent such as tetrahydrofuran,acetonitrile, methyl sulfoxide, and combinations thereof.

The next step illustrated in FIG. 2 is the solid state carbonizationprocess 108, which may comprise a multi-step heating regime. Asillustrated in FIG. 2, step 108 actually comprises four stepsillustrated by boxes 110, 112, 114, and 116. Each step in thecarbonization process will depend upon the DSC and TGA testing describedabove. Generally however, the step illustrated by box 110 comprises thestep of heating the dried monomers and optional acid to a firsttemperature at a predetermined rate of X° C./minute. Once the desiredtemperature is reached, the mixture is held at that temperature for apredetermined time period, as illustrated in box 112.

Thereafter, the material is heated to a second, typically highertemperature, at a rate of X° C./minute, as illustrated in box 114. Oncethe second desired temperature is reached, the mixture is held at thattemperature for a predetermined time period, as illustrated in box 116.After solid state carbonization is completed, the resulting carbonelectrode material is cooled slowly as illustrated in box 118.

The instant invention may be better understood from the examplesprovided below.

EXAMPLES

Each of the following examples provide X-ray diffraction analysis usinga CuK.sub.α source, where λ=1.54 Å source. Degrees 2θ are plotted on theabscissa, and intensity is plotted on the ordinate. Peak values werecalculated using Bragg's Law, i.e., d=λ/2 sin θ

EXAMPLE 1

To 1,3,5-benzenetricarbonyl trichloride (16.8 g) solution intetrahydrofuran was added pentaerythritol powder (8.04 g). The mixturewas dried and cured at 100° C. for 12 hours. The cured polymer was thencarbonized according to the following heating program in an inert gasatmosphere of argon: (1) 100° C. to 260° C. at 1° C./min; (2) hold at260° C. for 6 hours; (3) from 260° C. to 800° C. at 10° C./min; (4) holdat 800° C. for 1 hour. 6.41 g of carbon electrode material wascollected. FIG. 3 is an X-ray diffraction pattern for the material ofthis example, and shows a broad (002) peak centered at 4.03 Å. FIG. 3also shows an intensity peak ratio of ##EQU1## of 2.48. The reversiblelithium intercalation capacity of the material was 520 mAh/g.

EXAMPLE 2

To 1,3,5-benzenetricarbonyl trichloride (16.8 g) solution intetrahydrofuran was added pentaerythritol powder (8.04 g). The mixturewas dried and cured at 100° C. for 12 hours. The cured polymer was thencarbonized according to the following heating program in an inert gasatmosphere of argon: (1) 100° C. to 260° C. at 1° C./min; (2) hold at260° C. for 6 hours; (3) from 260° C. to 1000° C. at 10° C./min; (4)hold at 1000° C. for 1 hour. 6.02 g of carbon electrode material wascollected. FIG. 4 is an X-ray diffraction pattern for the carbonmaterial of this example, and shows a broad (002) peak in the range of4.06 to 3.90 Å, and centered at 4.00 Å. FIG. 4 also shows an intensitypeak ratio of ##EQU2## of 2.25. The reversible lithium intercalationcapacity of the material was 480 mAh/g.

EXAMPLE 3

To 1,3,5-benzenetricarbonyl trichloride (16.8 g) solution intetrahydrofuran was added pentaerythritol powder (8.04 g). The mixturewas dried and cured at 100° C. for 12 hours. The cured polymer was thencarbonized according to the following heating program in an inert gasatmosphere of argon: (1) 100° C. to 260° C. at 1° C./min; (2) hold at260° C. for 6 hours; (3) from 260° C. to 1100° C. at 10° C./min; (4)hold at 1100° C. for 1 hour. 6.00 g of carbon electrode material wascollected. FIG. 5 is an X-ray diffraction pattern for the material ofthis example, and shows a broad (002) peak centered at at 3.95 Å. FIG. 5also shows an instensity peak ratio of ##EQU3## of 2.48. The reversiblelithium intercalation capacity of the material was 470 mAh/g.

                  TABLE 1                                                         ______________________________________                                        Summary of experimental results for the carbon electrode material             made from 1,3,5-benzenetricarbonyl trichloride and pentaerythritol            Sample No.      1         2       3                                           ______________________________________                                        d-spacing of (002) (Å)                                                                    4.03      4.00    3.95                                        peak ratio of (002) to (100)                                                                  2.48      2.25    2.48                                        Capacity (mAh/g)                                                                              520       480     470                                         ______________________________________                                    

EXAMPLE 4

Terephthaloyl chloride (24.04 g) and pentaerythritol powder (8.00 g)were mixed in a ball mill. The mixture was placed in a ceramic crucibleand cured at 100° C. for 12 hours. The cured polymer was then carbonizedaccording to the following heating program in an inert gas atmosphere ofargon: (1) 100° C. to 260° C. at 1° C./min; (2) hold at 260° C. for 6hours; (3) from 260° C. to 800° C. at 10° C./min; (4) hold at 800° C.for 6 hours. 5.31 g of carbon electrode material were collected. FIG. 6is an X-ray diffraction pattern for the material of this example, andshows a broad (002) peak centered at at 3.89 Å. FIG. 6 also shows anintensity peak ratio of ##EQU4## of 2.65. The reversible lithiumintercalation capacity of the material was 470 mAh/g.

EXAMPLE 5

Terephthaloyl chloride (24.04 g) and pentaerythritol powder (8.00 g)were mixed in a ball mill. The mixture was placed in a ceramic crucibleand cured at 100° C. for 12 hours. The cured polymer was then carbonizedaccording to the following heating program in an inert gas atmosphere ofargon: (1) 100° C. to 260° C. at 1° C./min; (2) hold at 260° C. for 6hours; (3) from 260° C. to 1000° C. at 10° C./min; (4) hold at 1000° C.for 6 hours. 5.01 g of carbon electrode material was collected. FIG. 7is an X-ray diffraction pattern for the material of this example, andshows a broad (002) peak centered at at 3.95 Å. FIG. 7 also shows anintensity peak ratio of ##EQU5## of 2.25. The reversible lithiumintercalation capacity of the material was 430 mAh/g.

EXAMPLE 6

Terephthaloyl chloride (24.04 g) and pentaerythritol powder (8.00 g)were mixed in a ball mill. The mixture was placed in a ceramic crucibleand cured at 100° C. for 12 hours. The cured polymer was then carbonizedaccording to the following heating program in an inert gas atmosphere ofargon: (1) 100° C. to 260° C. at 1° C./min; (2) hold at 260° C. for 6hours; (3) from 260° C. to 1200° C. at 10° C./min; (4) hold at 1200° C.for 6 hours. 4.85 g of carbon electrode material was collected. FIG. 8is an X-ray diffraction pattern for the material of this example, andshows a (002) peak in the range of 4.92 to 3.79 Å, and centered at at3.95 Å. FIG. 8 also shows an intensity peak ratio of ##EQU6## of 2.25.The reversible lithium intercalation capacity of the material was 420mAh/g.

                  TABLE 2                                                         ______________________________________                                        Summary of experimental results for the carbon electrode                      material made from terephthaloyl chloride and pentaerythritol                 Sample No.      4         5       6                                           ______________________________________                                        d-spacing of (002) (Å)                                                                    3.93      3.89    3.86                                        peak ratio of (002) to (100)                                                                  2.65      2.25    2.25                                        Capacity (mAh/g)                                                                              470       430     420                                         ______________________________________                                    

EXAMPLE 7

Dimethyl isophthalate (19.4 g), pentaerythritol powder (6.80 g), andp-toluenesulfonic acid (1.94 g) were mixed and placed in a ceramiccrucible. After heating at 80° C. for about 30 minutes, the mixturebecame a viscous liquid. The liquid was subsequently cured at 130° C.for 12 hours. The cured polymer was carbonized according to thefollowing heating program in an inert gas atmosphere of argon: (1) 130°C. to 600° C. at 0.5° C./min; (2) from 600° C. to 1100° C. at 10°C./min; (4) hold at 1100° C. for 1 hour. 4.05 g of carbon electrodematerial was collected. FIG. 9 is an X-ray diffraction for the materialof this example, and shows a broad (002) peak centered at 3.95 Å. FIG. 9also shows an intensity peak ratio of ##EQU7## of 2.43. The reversiblelithium intercalation capacity of the material was 340 mAh/g.

EXAMPLE 8

Dimethyl terephthalate (19.4 g), pentaerythritol powder (6.80 g), andp-toluenesulfonic acid (1.94 g) were mixed and placed in a ceramiccrucible. After heating at 80° C. for about 30 minutes, the mixturebecame a viscous liquid. The liquid was subsequently cured at 130° C.for 12 hours. The cured polymer was carbonized according to thefollowing heating program in an inert gas atmosphere of argon: (1) 130°C. to 600° C. at 0.5° C./min; (2) from 600° C. to 1100° C. at 10°C./min; (4) hold at 1100° C. for 1 hour. 4.05 g of carbon was collected.FIG. 10 is an X-ray diffraction pattern for the material of thisexample, and shows a (002) peak centered at 3.83 Å. FIG. 10 also showsan intensity peak ratio of ##EQU8## of 2.38. The reversible lithiumintercalation capacity of the material was 330 mAh/g.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as defined by theappended claims.

What is claimed is:
 1. An electrode for an electrochemical cell, saidelectrode consisting of a carbon material being a pyrolytic by-productof at least a first and a second multifunctional organic monomer, andcharacterized by an X-ray difraction pattern in which the d-spacing ofthe (002) plane is between 3.89 and 4.20 Å.
 2. An electrode as in claim1, wherein said first multifunctional organic monomer is selected fromthe group of materials having the formulas: ##STR3##
 3. An electrode asin claim 1, wherein said second multifunctional organic monomer isselected from the group of materials having the formulas: ##STR4## whereR is selected from the groups of Cl, OH, H, OC_(n) H_(2n+1), wherein n=1to 10, and combination thereof.
 4. An electrode as in claim 1, whereinsaid carbon material is amorphous.
 5. An electrode as in claim 1,wherein said electrode is further characterized in that the ratio of thepeak at (002) to that at (100) is between 1 and
 5. 6. An electrode as inclaim 5, wherein said ratio is between about 2.25 and 2.75.